feat(rust,python,cli): support REGEXP and RLIKE pattern matching in SQL engine

[alexander-beedie]

• Adds polars SQL engine support for REGEXP and RLIKE pattern matching (functionally identical to each other).
• Minor tidy-up; inconsistent use of "Sql" vs "SQL" in the crate's object/func names - standardised on "SQL".
• Bonus unit test coverage for CASE … WHEN … ELSE … END and COALESCE.

Examples

Literal pattern match:

lf = pl.LazyFrame({
    "idx": [0, 1, 2, 3, 4],
    "val": ["ABC", "abc", "000", "A0C", "a0c"],
})
with pl.SQLContext(test_data=lf, eager_execution=True) as ctx:
    print( ctx.execute("SELECT * FROM test_data WHERE val NOT REGEXP '.*c$'") )

# shape: (3, 2)
# ┌─────┬─────┐
# │ idx ┆ val │
# │ --- ┆ --- │
# │ i64 ┆ str │
# ╞═════╪═════╡
# │ 0   ┆ ABC │
# │ 2   ┆ 000 │
# │ 3   ┆ A0C │
# └─────┴─────┘

Expression pattern match (new: not available through the existing ~, ~*, etc regex ops):

lf = pl.LazyFrame({
    "idx": [0, 1, 2, 3, 4],
    "val": ["ABC", "abc", "000", "A0C", "a0c"],
    "pat": ["^A", "^A", "^A", r"[AB]\d.*$", ".*xxx$"],
})
with pl.SQLContext(test_data=lf, eager_execution=True) as ctx:
    print( ctx.execute(f"SELECT idx,val FROM test_data WHERE val REGEXP pat") )

# shape: (2, 2)
# ┌─────┬─────┐
# │ idx ┆ val │
# │ --- ┆ --- │
# │ i64 ┆ str │
# ╞═════╪═════╡
# │ 0   ┆ ABC │
# │ 3   ┆ A0C │
# └─────┴─────┘